Gee I drive a rolling hilly type back road to and from work, no real stop and go traffic, and I just got 16mpg with my 6 last week. Drrrrrr! I wonder what I should be getting!
Youāre driving a V12, with a displacement of 5.3L. Fuel economy? Just be glad that it got you home 
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Alright, 16mpg combined is much different than pure city. Iāve been driving mine in Seattle traffic, lots of stop lights, stop and go. So IMO I should be getting 12mpg, not 10.5mpg. Timing is well advanced, and I disconnected the switch that retards timing when cold.
Iāll do an ITU soon and see what happens.
Same same and itās the best it can do.
But for the sake of comparison:
A friend of mine has a 5.0L Range Rover Supercharged and he gets (at the same driving conditions) 12 mpg - 20lt/100kmā¦
He also has a W12 VW Phaeton, he gets a miserable 9.5mpg - 25lt/100kmā¦!
And thatās not city driving.
One time I got 21 mpg - 11lt/100km but that was cruising on a flat empty road all day long at 80km/h.
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Yeah my 98 f150 with a 4.6 gets 14-16 on a decent day. My 1980 Malibu with a 355 would be lucky to get 17mpg if I took it easy, but that was with non ethenol fuel. Like Iāve said to people, if I was concerned about the mpg, I wouldnāt have bought the stuff I have. Lol 
You will get 10MPG if you drive it like an @sshole. Otherwise on the flats in Florida I usually get 15mpg avg.
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Very little city driving, nice long country drives my 6 litre V12 regularly returns 10.4 litres per 100km.
Drove my 94 4.0 on a 400 mile trip this weekend. I received 27 mpg on the highway doing 65-70 with A/C on. Combined was 23 mpg. Iām very pleased with those numbers. Not a V12 but still purred like a tomcat in a creamery!
Gordon
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Wow! Iād like to get to go for a nice trip and see how mine does. Mine runs good, but I think it could use an analysis or something to get stuff adjusted perfectly
We had pre HE 8 to 10 maybe 12 was a stretch, then moved into the HE 12 to 16 .
The second HE later was about the same.
These were all new.
These cars are over 4,000 pounds, what brings up gas mileage?
Its a grand tourerā¦blow some carbon baby!
Our last JCNA concourse I had a gent go over this same subject.
Other than running at peak performance for drivability, WHY care about mileage, unless there are issues.ā
gtjoey1314
Trade it in for a prius if we want 30 mpgā¦haaaa
having fun.
Asking damn fool questions about V12 Jags and fuel economy? If you donāt like the answerā¦
My original questions about fuel economy were based on if my engine is running correctly or not. At 10.5mpg, sounds like I still have an issue.
I understandā¦
We have a 40 year old engine in a 40 year old carā¦Until something falls off UNLESS you hear knocking, black smoke, blue smoke or head gasket leaks.
Its an old car.
THE GAS IS PLAYING A LARGE ROLL IN YOUR PERFORMANCEā¦
Good luck, drive it till it blows.
Having fun with you Greg.
How many miles did you drive this year?
Cats could be plugged as well.
I thought the cats were plugged tooā¦Iāve removed them, and it is still 10.5mpg.
You are right, that Iām not burning oil, nor hearing any strange noises, so all is probably well.
But I am a perfectionist in that I want to get this engine at optimum health, even though itās 31 years old.
Many here say I just need an Italian Tune Up, which is probably true. Car sat for 7 years before I bought it, ran rich for the year prior with PO. But I am not ready to take it above 50mph, due to cracked tires and Iām still renewing/checking the suspension.
Ahhh now we are getting somewhere.
Removing cats on a pre fuel injection car or early obd2 make a difference hp gain.
You might be hurting your mileage without the higher heat temp and back pressure that the cats provide or simply , your dumping more fuel because the heat cycle is not hot enough to go to optimum performance.
Removing cats today is not a good thing.
its more back pressure head efficiency than anything else.
Since it sat for 100 years, go to any performance store and get a 5 gallon can of 94 pure octaine unleaded. It will cost 75 dollars.
You would thing you rebuilt the engine!
Put back stock cats or free flow, stock would be better.
Good luck young man!
gtjoey1314
I have a data point from the weekend from my 85 XJS.
It is on the way to as efficient as possible with a 5 speed manual and full computer control over engine managementā¦which I am still playing with. A reasonable open road run at between 110-120kph for about 200 km with some spirited passing manoeuvres yielded 11.9km/litre which is 24 mpg (imp) and 20mpg(USA).
This was locked in 5th gear for most of the time so not heating trans fluid.
I recall getting about 22-23 mpg(imp) (18mpg (USA) on the same trip with the original setup.
I am also running 180F thermostats, so the car usually runs 190F in stop and go traffic, and 180F when cruising. A bit lower than I think the stock thermostats at 195F.
Once I get some Euro Downpipes, I will have some cats welded into the intermediate pipes. If nothing else, I donāt like the exhaust smell nowā¦My XJS smells like a 1960ās Pontiac!
Gregā¦ Kilbert was right ā¦
With a
too cold thermostat
no cats
bad gasā¦
Your dumping fuel as its cycling cold and richening up the mixture!
It theory also retarding or advancing the timing to a drastic degree to compensate.
Thatās why it smells like a 60s Pontiac, you just dumping fuelā¦
Pull one plug it should be pretty sooty.
- put stock cats.
- good fuel
- new plugs ngK go back to STOCK thermostat.
YOU WILL NOTICE A BIG DIFFERENCE.
GTJOEY1314
Matt has a wonderful set up but in the states it didnāt exist, plus the overdrive or 5th drops rpms by at least 400 to 500 rpm at cruising speeds. In the states the OLD and bullet proof gm400 was plugging along, weight and limited gearsā¦
It looks like the OEM thermostats are indeed 195F. So I am running 15F below standard operating temps at 180F.
But I thought it would help prevent dropped valve seats, as I never get over 200F now.
Greg,
IMHO running with cooler than spec thermostats is not doing anything to help you and may possibly be hurting your fuel economy. This is particularly true in your cooler NW USA climate. Water boils at 212F at sea level atmospheric pressure. Running around 190F-200F in a properly running and serviced engine (particularly coolant and distributor timing) is not going to cause dropped valve seats. However, it is quite possible that it will improve your fuel economy.
Paul